In recent years, the demands on biomaterials essential to the biomedical industries have been increased as a result of a rapid advance in public welfare, health improvement, and medical science. In particular, needs have continued to exist for the development of biomaterials which have specific applications in various areas such as biosenser, biotechnology, materials for orthopedics, sanitary supplies, matrices for drug delivery system and artificial organs. The biomaterials cover metals, ceramics and polymers which can be applied to the said areas, and among them, polymers having the advantages of lightness and flexibility have been regarded as a representative material.
In general, the polymers used for biomedical materials are classified depending on their degradation properties into biodegradable polymer and nondegradable polymer. Nondegradable polymer has been used as surgery materials, although it has a disadvantage of undergoing removal after surgery. Still, biodegradable polymer has merit over the nondegradable polymer in a sense that it can be easily degraded by water or enzyme.
Studies on the various polymers which can be used for medical purpose have been actively carried out, since a biodegradable polymer with polylactide and polyglycolic acid was approved in FDA. However, there are strong reasons for exploring alternative biomaterials in order to meet various mechanical properties for specialized medical purpose such as materials for orthopedics, implants and matrices for drug delivery system while possessing a biocompatible character.
Under the circumstances, biocompatible aliphatic polyesters, such as, polylactide, polyglycolide and polycaprolactone have been suggested as promising biodegradable polymers. However, the said polymers tend to adhere to cells or proteins due to their low hydrophilicity and cause numerous problems when they are applied to a living body. Accordingly, in order to attenuate the said hydrophobic nature, new approaches to the copolymerization of hydrophilic polyether have been made in the art.
For example, Lee et al. disclosed that the protein adhesion was significantly suppressed when the hydrophilic polymer, particularly with, polyethylene oxide is located on the side chain(see: J. H. Lee et al., J. Biomed. Materials Res., 23:351(1989)). Furthermore, a ring-opening polymerization of L-lactide and ethyleneoxide was successfully carried out to obtain a random copolymer by using various catalysts(see: X. Chen et al., Macromolecules, 30:4295(1997)), although it is very difficult to regulate the amount of monomers, since the ratio of product is not correlated with molar ratio of the added monomers. And further, researches on the block-copolymers of monomer such as caprolactone, lactide, and glycolide or homopolymeric blending with the copolymers have been progressed in the art.
On the other hand, U.S. Pat. No. 5,741,881 discloses that blood compatibility was dramatically improved by grafting polyethyleneoxide onto the side chain of polyurethane, although it turned out unsuitable for biomedical materials owing to its abrasive effect and undegradable property in a living body. Furthermore, U.S. Pat. No. 5,548,035 teaches that: polyethyleneglycol and polylactide having various terminal groups, copolymer of polylactide-polyglycolide or polycaprolactone are copolymerized to give multi-block copolymers, which show non-toxic and biodegradable properties; and, therefore, they can be successfully used for matrices for drug delivery system.
Besides, aliphatic polyester having a structure to maximize the effect of polyether substitution by positioning hydrophilic groups on the surface, which is suitable for biomedical polymers, have been successively considered in the art.